This application is a 371 application of PCT/JP98/04834 filed Oct. 26, 1998.
The present invention relates to novel liquid crystalline compounds and liquid crystal compositions. More specifically, the invention relates to liquid crystalline compounds having 2,3-difluorophenyl moiety, liquid crystal compositions comprising the compound, and liquid crystal display devices fabricated by using the liquid crystal composition.
Liquid crystal display devices fabricated by using liquid crystalline compounds (the term xe2x80x9cliquid crystalline compoundsxe2x80x9d is used in this specification as a generic name for the compounds which exhibit a liquid crystal phase and the compounds which do not exhibit a liquid crystal phase but are useful as a component of liquid crystal compositions) are widely being used for the display of computers, television sets, and the likes.
For the purpose of reducing the power consumption and decreasing the leakage of electromagnetic wave, liquid crystal compositions are required to lower their driving voltage. Driving voltage (threshold voltage) is known to be a function of dielectric anisotropy value and elastic constant according to the following equation (M. F. Leslie, Mol. Cryst. Liq. Cryst., 12, 57 (1970)):
xe2x80x83Vth=xcfx80(K/xcex50xcex94xcex5)xc2xd
wherein Vth represents a threshold voltage, xcex50: a dielectric constant in vacuum, K: an elastic constant, and xcex94xcex5: a dielectric anisotropy, respectively.
That is, it can be understood that in order to lower driving voltage, it is necessary 1) to increase dielectric anisotropy value or 2) to decrease elastic constant.
It is generally considered to be difficult to adjust the value of the elastic constant of liquid crystalline compounds, and thus a means in which the dielectric anisotropy value is increased is principally adopted to lower the driving voltage. Accordingly, novel liquid crystalline compounds having a large dielectric anisotropy value are long-expected.
From some time ago, a characteristic of narrow visual angle is considered to be a most serious problem to liquid crystal display devices, and various display modes have been proposed in recent years for the purpose of improving the narrow visual angle. In-plane switching (IPS) display devices proposed in 1995 greatly widened the visual angle compared with conventional display devices (Liquid Crystal Conference in Japan 2A07 (1995), ASIA DISPLAY ""95, 557 (1995) and ASIA DISPLAY ""95, 707 (1995)).
Also, in 1997, an attempt was reported in which a vertical alignment (VA) cell was used (SID 97 DIGEST, 845 (1997)), and the display devices of this mode are considerably wide in visual angle compared with conventional display devices.
In either mode of IPS and VA, characteristics required of liquid crystal compositions are
1) a negative and large dielectric anisotropy value (xcex94xcex5) for lowering driving voltage, and
2) a small optical anisotropy value (xcex94n) for keeping xcex94nxc2x7d (product of optical anisotropy value multiplied by cell thickness) at an optimum value.
However, compounds having simultaneously a negative and large dielectric anisotropy value and a small optical anisotropy value are heretofere unknown, and thus novel liquid crystalline compounds having such characteristics have been long-expected.
As compounds having a negative and large dielectric anisotropy value and a comparatively small optical anisotropy value, the compound of the following formula (13) is known (V. Reifffenrath et al., Liq. Cryst., 5 (1), 159 (1989)). It is reported that the dielectric anisotropy value of this compound is (xcex94xcex5=xe2x88x924.1) and optical anisotropy value is (xcex94n=0.18). 
However, the dielectric anisotropy value of the compound can not be said to be sufficiently large, and a satisfactory lowering of driving voltage was unable to actualize.
As compounds having a negative dielectric anisotropy value, the terphenyl compound of the formula (14) is known (J. Chem. Soc. Perkin Trans. II 2, 2041 (1989)). This compound is narrow (10.5xc2x0 C.) in the temperature range showing nematic phase and exhibits smectic phase in a wide temperature range (50.5xc2x0 C.). Further, terphenyl compounds were extremely large in optical anisotropy value in general and were unsuitable as component of liquid crystal compositions for IPS mode or VA mode. 
The phenomenon that dielectric anisotropy value is increased in negative when fluorine atom is introduced at a lateral position of a phenylene group which constitutes the skeleton of liquid crystalline compounds is well known to a person skilled in the art. On the other hand, the order parameters of liquid crystalline compounds are decreased by the introduction of fluorine atom to the lateral position. Dielectric anisotropy value and optical anisotropy value are regarded as functions of the order parameters (W. Maier and G. Meier, Z. Naturf. (a), 16 262 (1961)), and the decrease of the order parameters caused by the introduction of fluorine atom will bring about the decrease in dielectric anisotropy value. Accordingly, the introduction of fluorine atom to the lateral position does not necessarily produce a large increase in negative dielectric anisotropy value (Theory of Maier and Meier).
In view of the several characteristics described above and required of liquid crystal compositions, an object of the present invention is to provide liquid crystalline compounds having a negative and extremely large dielectric anisotropy value and a small optical anisotropy value at the same time, to provide liquid crystal compositions comprising the compound, and to provide liquid crystal display devices fabricated by using the liquid crystal composition.
As a result of diligent research and development conducted by the present inventors to solve the problems described above, it has been found that the liquid crystalline compounds expressed by the general formula (1) have desired characteristics, leading to the accomplishment of the present invention. 
wherein Ra and Rb each independently represent a straight chain or branched alkyl group or alkoxy group having 1 to 10 carbon atoms, or a straight chain or branched alkenyl group or alkynyl group having 2 to 10 carbon atoms; ring A1 represents cyclohexane-1,4-diyl in which ring not-adjacent any methylene group may be replaced by xe2x80x94Oxe2x80x94; ring A2 represents 2,3-difluoro-1,4-phenylene in which phenylene hydrogen atoms at 5-position and 6-position may each independently be replaced by fluorine atoms, but there is not a case wherein both of the hydrogen atoms are simultaneously replaced; Z1 and Z2 each independently represent single bond or xe2x80x94CH2CH2xe2x80x94; Xa, Xb, Xc, and Xd each independently represent hydrogen atom, fluorine atom, or chlorine atom, but at least one of Xa, Xb, Xc, and Xd is fluorine atom or chlorine atom; and any atom which constitutes the compound may be replaced by its isotope.
Among the liquid crystalline compounds expressed by the general formula (1), the compounds which exhibit particularly preferable characteristics are those expressed by one of the following general formulas (1-1) to (1-18) 
In the general formula (1), Ra and Rb are alkyl groups, alkoxy groups, alkoxyalkyl groups, alkoxyalkoxy groups, alkenyl groups, or alkynyl groups. Compounds in which Ra and Rb are alkyl groups, alkoxy groups, alkoxyalkyl groups, or alkoxyalkoxy groups are chemically stable, and the compounds in which Ra and Rb are alkenyl groups or alkynyl groups exhibit a slightly large optical anisotropy.
Also, the compounds in which Ra and Rb are alkyl groups, alkoxy groups, or alkenyl groups are preferable since they have a low viscosity. Further, when used for display devices of IPS mode or VA mode, the compounds in which Ra and Rb are alkyl groups or alkoxy groups are optimum since a high chemical stability and a small optical anisotropy value are required in such case.
While Z1 and Z2 are each independently single bond or xe2x80x94CH2CH2xe2x80x94, the compounds in which one of Z1 and Z2 is single bond have a lower viscosity and the compounds in which one of them is xe2x80x94CH2CH2xe2x80x94 have nematic phase in a wider temperature range. Group xe2x80x94CH2CH2xe2x80x94 is preferably introduced to Z1, and when it is introduced to Z2, compounds are provided of which the upper limit of the temperature range exhibiting nematic phase is slightly low.
While Xa, Xb, Xc, and Xd are each independently hydrogen atom, fluorine atom, or chlorine atom, at least one of Xa, Xb, Xc, and Xd is a halogen atom. In the halogen atoms, fluorine atom is preferable, and when it is chlorine atom, compounds having a slightly high viscosity are provided.
Number of the halogen atom is preferably 1, 2, or 3, and 1 or 2 is desirable to obtain compounds having a low viscosity.
While ring A1 is cyclohexane-1,4-diyl, 1,3-dioxane-2,5-diyl, or tetrahydropyran-2,5-diyl, cyclohexane-1,4-diyl is optimum to obtain compounds having a low viscosity. Compounds in which 1,3-dioxane-2,5-diyl is introduced to the position of ring A1 have a small elastic constant value (K), and are preferable since they lower the driving voltage of liquid crystal display devices of TN mode including IPS mode or VA mode.
Further, any atom which constitutes the compounds of the present invention may be replaced by its isotope since the compounds in which the atom is replaced by the isotope exhibit the same characteristics.
Compounds of the present invention expressed by the general formula (1) can be produced by using known procedures of organic synthetic chemistry in a suitable combination. The known procedures of organic chemistry can be found by consulting such books as organic Synthesis, Organic Reactions, and Shin-Jikken Kagaku Kouza (Course of New Chemical Experiment), and their typical examples are shown below. 
Compounds expressed by the general formula (1) wherein Z2 is single bond can be produced, for instance, by the following method. That is, iodized benzene derivative (16) can be obtained by reacting halogenated benzene derivative (15) with n- or sec-butyl lithium and then reacting with iodine. Increase of yield can be expected by selecting the type of butyl lithium to be used depending on the position and number of halogen atom in halogenated benzene derivative (15).
Compounds of the general formula (1) can be produced by reacting the iodized benzene derivative (16) with phenyl boric acid derivative (17) in the presence of a catalyst to perform a cross-coupling reaction. While the catalyst to be used is preferably Pd or Ni type, any catalyst may be. used so far as it makes the reaction smoothly proceed. The phenyl boric acid derivative (17) can be produced by reacting a corresponding phenyl magnesium halide derivative with an boric acid ester at a low temperature.
Further, the compounds of the general formula (1) can be produced by converting halogenated benzene derivative (15) into an organic zinc compound and then reacting with phenyl halide derivative (18) in the presence of a catalyst. While the catalyst to be used is preferably Pd or Ni type, any catalyst may be used so far as it makes the reaction smoothly proceed.
Halogenated benzene derivative (15) can be produced by reacting cyclohexanone derivative (26) or aldehyde derivative (27) with a Grignard reagent (28) and then subjecting to a dehydration reaction and hydrogenation reaction in the same manner as described above. 
Compounds expressed by the general formula (1) wherein Z2 is xe2x80x94CH2CH2xe2x80x94 can be produced by the method described below. That is, compound (19) can be produced by reacting the halogenated benzene derivative (15) described above with n- or sec-butyl lithium and then with ethylene oxide to obtain an alcohol derivative, and subsequently halogenating (preferably brominating or iodinating) the alcohol derivative. Compounds of the general formula (1) can be produced by reacting the compound (19) with metal lithium under ultrasonic radiation to convert into an organic lithium compound, reacting in-situ with zinc chloride to form a zinc compound, and then reacting it with the phenyl halide derivative (18) described above in the presence of a catalyst. While the catalyst to be used is preferably Pd or Ni type, a Pd catalyst of zero valent is particularly preferable.
Compounds of the general formula (1) can also be obtained by reacting halogenated benzene derivative (15) with n- or sec-butyl lithium and subsequently with ethylene oxide to obtain an alcohol derivative, oxidizing it to convert into aldehyde derivative (20), reacting it with a Grignard reagent (21) which can be prepared from phenyl halide derivative (18), and further subjecting it to a dehydration reaction and a hydrogenation reaction in turn. 
Compounds expressed by the general formula (1) wherein ring A1 is cyclohexane-1,4-diyl can preferably be produced by the following method. That is, biphenyl derivative (24) can be produced by reacting halogenated phenyl bromide or halogenated phenyl iodide (22) with phenyl boric acid derivative (23) in the presence of a catalyst to perform a cross-coupling reaction. Compounds of the general formula (1) can be obtained by reacting biphenyl derivative (24) with n- or sec-butyl lithium and subsequently with cyclohexanone derivative (25), and then subjecting it to a dehydration reaction and hydrogenation reaction as in the case described above.
In order to introduce 1,3-dioxane-2,5-diyl or tetrahydropyran-2,5-diyl to the position of ring A1, it is sufficient to follow the method disclosed by H. M. Vorbrodt, R. Eidenschink et al. (H. M. Vorbrodt, J. Prakt, Chem., 323, 902 (1981), R. Eidenschink, DE-OS3306960).
Since the liquid crystalline compounds of the present invention obtained by such a method exhibit a negative and extremely large dielectric anisotropy, low voltage driving of liquid crystal display devices can be actualized.
Also, since the liquid crystalline compounds of the present invention are sufficiently stable physically and chemically under conditions wherein liquid crystal display devices are ordinarily used, are readily mixed with various liquid crystal materials, and have an extremely excellent miscibility even at low temperatures, the compounds are remarkably excellent as component of nematic liquid crystal compositions.
These compounds have a negative and large dielectric anisotropy value and a comparatively small optical anisotropy value at the same time, and can preferably be used as component of liquid crystal compositions for IPS mode or VA mode in particular.
Liquid crystal compositions of the present invention are described below. Liquid crystal compositions of the present invention preferably comprise at least one compound expressed by the general formula (1) in the ratio of 0.1 to 99.9% by weight to develop excellent characteristics, and the amount is more desirably 1 to 60% by weight.
Still more desirably, the liquid crystal compositions of the present invention are completed by mixing a compound arbitrarily selected from the group consisting of the compounds expressed by one of the general formulas (2) to (12) depending on the purpose of the liquid crystal compositions, in addition to a first component comprising at least one compound expressed by the general formula (1). 
wherein R1 represents an alkyl group having 1 to 10 carbon atoms in which group not-adjacent any methylene group may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen atom in the group may be replaced by fluorine atom; X1 represents fluorine atom, chlorine atom, xe2x80x94OCF3, xe2x80x94OCF2H, xe2x80x94CF3, xe2x80x94CF2H, xe2x80x94CFH2, xe2x80x94OCF2CF2H, or xe2x80x94OCF2CFHCF3; L1 and L2 each independently represent hydrogen atom or fluorine atom; Z3 and Z4 each independently represent xe2x80x94CH2CH2xe2x80x94, xe2x80x94(CH2)413 , xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or single bond; ring B represents cyclohexane-1,4-diyl or 1,3-dioxane-2,5-diyl, or 1,4-phenylehe in which hydrogen atom may be replaced by fluorine atom; ring C represents cyclohexane-1,4-diyl, or 1,4-phenylene in which hydrogen atom may be replaced by fluorine atom; and any atom which constitutes these compounds may be replaced by its isotope, 
wherein R2 and R3 each independently represent an alkyl group having 1 to 10 carbon atoms in which group not-adjacent any methylene group may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen atom in the group may be replaced by fluorine atom; X2 represents xe2x80x94CN or xe2x80x94Cxe2x80x94xe2x89xa1Cxe2x80x94CN; ring D represents cyclohexane-1,4-diyl, 1,4-phenylene, 1,3-dioxane-2,5-diyl, or pyrimidine-2,5-diyl; ring E represents cyclohexane-1,4-diyl, 1,4-phenylene in which hydrogen atom may be replaced by fluorine atom, or pyrimidine-2,5-diyl; ring F represents cyclohexane-1,4-diyl or 1,4-phenylene; Z5 represents xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, or single bond; L3, L4, and L5 each independently represent hydrogen atom or fluorine atom; b, c, and d are each independently 0 or 1; and any atom which constitutes these compounds may be replaced by its isotope, 
wherein R4 and R5 each independently represent an alkyl group having 1 to 10 carbon atoms in which group not-adjacent any methylene group may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen atom in the group may be replaced by fluorine atom; ring G and ring I each independently represent cyclohexane-1,4-diyl or 1,4-phenylene; L6 and L7 each independently represent hydrogen atom, cyano group, or fluorine atom, but there is not a case wherein L6 and L7 represent hydrogen atom at the same time; Z6 and Z7 each independently represent xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, or single bond; and any atom which constitutes these compounds may be replaced by its isotope, 
wherein R6 and R7 each independently represent an alkyl group having 1 to 10 carbon atoms in which group not-adjacent any methylene group may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen atom in the group may be replaced by fluorine atom; ring J, ring K, and ring M each independently represent cyclohexane-1,4-diyl or pyrimidine-2,5-diyl, or 1,4-phenylene in which hydrogen atom may be replaced by fluorine atom; Z8 and Z9 each independently represent xe2x80x94CH2CH2xe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or single bond; and any atom which constitutes these compounds may be replaced by its isotope.
Compounds expressed by one of the general formulas (2) to (9) are a second component, and the compounds expressed by one of the general formulas (10) to (12) are a third component in the liquid crystal compositions of the present invention, respectively.
As preferable examples of the compounds used in the liquid crystal compositions of the present invention and expressed by one of the general formulas (2) to (4), the compounds of the following general formulas can be mentioned: 
wherein R1 and X1 have the same meaning as described above.
Compounds expressed by one of the general formulas (2) to (4) have a positive dielectric anisotropy value, are remarkably excellent in thermal stability and chemical stability, and are extremely useful especially when liquid crystal compositions for AM (active matrix), particularly for TFT (thin film transistor) of which a high reliability such as a high voltage holding ratio and a large specific resistivity is required are produced.
When liquid crystal compositions for AM are produced, the compound expressed by one of the general formulas (2) to (4) can be used in the range of 0.1 to 99.9% by weight based on the total amount of liquid crystal composition, the amount is preferably 10 to 97% by weight and more desirably 40 to 95% by weight. Besides, the compound expressed by one of the general formulas (10) to (12) may further be added for the purpose of adjusting viscosity.
Even when liquid crystal compositions for STN or TN are produced, the compound expressed by one of the general formulas (2) to (4) can be used, but its amount is preferably less than 50% by weight.
As preferable examples of the compounds used in the liquid crystal compositions of the present invention and expressed by the general formula (5) or (6), the compounds of the following general formulas can be mentioned: 
wherein R2, R3, and X2 have the same meaning as described above.
Compounds expressed by the general formula (5) or (6) have a large positive dielectric anisotropy value, and are used particularly for the purpose of lowering threshold voltage of liquid crystal compositions. Also, they are used for the purpose of adjusting optical anisotropy value and widening nematic range such as raising clearing point. Further, they are used for the purpose of improving the steepness of voltage-transmittance curve of liquid crystal compositions for STN or TN.
Compounds expressed by the general formula (5) or (6) are particularly useful when liquid crystal compositions for STN or TN are produced.
When the amount of the compound expressed by the general formula (5) or (6) is increased in liquid crystal compositions, threshold voltage of liquid crystal compositions lowers but viscosity rises. Accordingly, it is advantageous to use a large amount of the compound since liquid crystal display devices can be driven at a low voltage so far as the viscosity of liquid crystal compositions satisfies a required value. When liquid crystal compositions for STN or TN are produced, the compound expressed by the general formula (5) or (6) can be used in the range of 0.1 to 99.9% by weight based on the total amount of liquid crystal composition, and the amount is preferably 10 to 97% by weight and more desirably 40 to 95% by weight.
As preferable examples of the compounds used in the liquid crystal compositions of the present invention and expressed by one of the general formulas (7) to (9), the compounds of the following general formulas can be mentioned: 
wherein R4 and R5 have the same meaning as described above.
Compounds expressed by one of the general formulas (7) to (9) have a negative dielectric anisotropy. Since the compounds expressed by the general formula (7) are two rings compounds, they are used principally for the purpose of adjusting threshold voltage, adjusting viscosity, or adjusting optical anisotropy value. Compounds expressed by the general formula (8) are used for the purpose of widening nematic range such as raising ;clearing point and for the purpose of adjusting optical anisotropy value. Compounds expressed by the general formula (9) are used for the purpose of adjusting optical anisotropy value.
Compounds expressed by one of the general formulas (7) to (9) are used principally for liquid crystal compositions having a negative dielectric anisotropy value. When the amount of the compound expressed by one of the general formulas (7) to (9) is increased in liquid crystal compositions, threshold voltage of liquid crystal compositions lowers but viscosity rises. Accordingly, it is desirable to use the compound in a small amount so far as the threshold voltage satisfies a required value. However, since the absolute value of the dielectric anisotropy of the compounds expressed by one of the general formulas (7) to (9) is smaller than 5, low voltage driving sometimes becomes impossible when the amount of the compounds becomes less than 40% by weight.
When liquid crystal compositions for TFT having a negative dielectric anisotropy value are produced, it is preferable to use the compound expressed by one of the general formulas (7) to (9) in a range of more than 40% by weight based on the total amount of liquid crystal composition and 50 to 95% by weight is preferable.
Also, for the purpose of improving the steepness of voltage-transmittance curve by controlling elastic constant, a compound expressed by one of the general formulas (7) to (9) is sometime added to liquid crystal compositions having a positive dielectric anisotropy value. In this case, the amount of the compound expressed by one of the general formulas (7) to (9) is preferably less than 30% by weight.
As preferable examples of the compounds used in the liquid crystal compositions of the present invention and expressed by one of the general formulas (10) to (12), the compounds of the following general formulas can be mentioned: 
wherein R6 and R7 have the same meaning as described above.
Compounds expressed by one of the general formulas (10) to (12) are small in the absolute value of dielectric anisotropy and the value is close to zero. Compounds expressed by the general formula (10) are used principally for the purpose of adjusting viscosity or adjusting optical anisotropy value. Compounds expressed by the general formula (11) or (12) are used for the purpose of widening nematic range such as raising clearing point or for the purpose of adjusting optical anisotropy value.
When the amount of the compound expressed by one of the general formulas (10) to (12) is increased in liquid crystal compositions, threshold voltage of the liquid crystal compositions rises and viscosity lowers. Accordingly, it is desirable to use the compound in a large amount in the range wherein the threshold voltage of the liquid crystal compositions satisfies a required value. When liquid crystal compositions for TFT are produced, the amount of the compound expressed by one of the general formulas (10) to (12) is preferably less than 40% by weight in liquid crystal compositions and the amount is more desirably less than 35% by weight. Further, when liquid crystal compositions for STN or TN are produced, the amount of the compound expressed by one of the general formulas (10) to (12) is preferably less than 70% by weight in liquid crystal compositions and the amount is more desirably less than 60% by weight.
In liquid crystal compositions for STN, TFT, or others, an optically active compound is usually added for the purpose of inducing helical structure of liquid crystals to adjust required twist angle and to prevent reverse twist. Even in the liquid crystal compositions of the present invention, any of known optically active compounds can be added for such purposes. As examples of preferable optically active compounds, the compound of the following formulas can be mentioned: 
Usually, these optically active compounds are added to the liquid crystal compositions of the present invention to adjust the pitch of the twist. Pitch of the twist is preferably adjusted in the range of 40 to 200 xcexcm in the case of liquid crystal compositions for TFT or TN, and preferably adjusted in the range of 6 to 20 xcexcm in the case of liquid crystal compositions for STN. In the case of liquid crystal compositions for bistable TN, it is preferably adjusted in the range of 1.5 to 4 xcexcm. Besides, two or more kind of optically active compounds may be added for the purpose of adjusting the dependency of the pitch on temperature.
Liquid crystal compositions of the present invention can be used as ones for GH (guest-host) mode by adding a dichroic dye such as merocyanine type, styryl type, azo type, azomethine type, azoxy type, quinophthalone type, anthraquinone type, and tetrazine type thereto. Further, the liquid crystal compositions can be used as NCAP which is prepared by the microencapsulation of a nematic liquid crystal, or as liquid crystal compositions for polymer dispersed liquid crystal display devices (PDLCD) represented by polymer net work liquid crystal display devices (PNLCD) prepared by forming a polymer of three-dimensional reticulated structure in a liquid crystal. Still further, the liquid crystal compositions of the present invention can be used as ones for ECB (electrically controlled birefringence) mode or dynamic scattering (DS) mode.
Liquid crystal compositions of the present invention can be produced by conventional methods. Generally, a method in which various components are dissolved in one another at a high temperature is adopted.